­NORTH AMERICA’S MIDCONTINENT RIFT: WHEN RIFT MET LIP

Rifts and flood basalts are major features associated with continental volcanism that differ in geometry and origin. Rifts are segmented linear depressions filled with sedimentary and igneous rocks that form by extension and often evolve into plate boundaries. Flood basalts, a class of Large Igneous Provinces (LIPs), are broad regions of extensive volcanism due to sublithospheric processes. Typical rifts are not associated with flood basalts, and typical flood basalts are not associated with significant crustal extension and faulting. North America’s Midcontinent Rift (MCR) is an unusual combination, because its 3000-km length formed as part of a continental breakup event 1.1 Ga, but contains an enormous volume of igneous rocks, mostly flood basalt typical of a LIP. We show that the MCR volcanic rocks are significantly thicker than those in other flood basalts, due to their deposition in a narrow rift rather than across a broad region, giving the MCR the geometry of a rift but the magma volume and composition of a LIP. Numerical structural modeling based on seismic reflection data shows that the LIP volcanics were deposited during two phases: an initial rift phase in which the flood basalts filled a fault-controlled extensional basin, and a post-rift phase in which LIP volcanics and later associated sediments were deposited in a thermally subsiding sag basin without associated faulting. The restriction of extension to a single normal fault, steeply inward-dipping rift shoulders with sharp hinges, and persistence of volcanism after rifting ended gave rise to a deep flood-basalt filled rift geometry not observed in other presently-active or ancient rifts. The unusual coincidence of a LIP with a rift arose when a new rift associated with continental breakup interacted with a mantle plume or overrode anomalously hot or fertile upper mantle.